Shaft furnaces

ABSTRACT

An apparatus for the calcination of a mineral, for example lime, magnesite or dolomite which consists of a fuel oil-fired furnace provided with two parallel shafts. Each of the shafts communicate with one or more gas bypass passages one of which is common to both shafts, which enables the downwardly moving charge to be isolated from the upwardly flowing furnace gases at a point in the furnace where a high degree of temperature uniformity is necessary in the charge.

United States Patent i [72] inventor Edward CharlesJope Chambers, High Street, Epsom, Surrey,

England [2| 1 Appl. No. 809,876 [22] Filed Mar. 24,1969 [45] Patented Aug. 24, 1971 [32] Priority Mar. 29, 1968 [33!] Great Britain [54] SHAFT FURNACES 10 Claims, 6 Drawing Figs.

[52] US. Cl a.

v 34/174, 263/53 [51] Int. Cl F27b 1/00 [50] Field of Search 263/29, 30, 53; 266/25; 34/174 Catagas Limited, National Provincial Bank [56] .v References Cited UNITED STATES PATENTS 2,650,814 9/1953 l-lowden 263/29 3,l75.302 3/l965 Retnli et ul 34/! 74 FOREIGN PATENTS l,l24,863 5/1954 Germany 263/30 Primary Examiner-John J. Camby Attorneywenderoth, Lind & Ponack ABSTRACT: An apparatus for the calcination of a mineral, for example lime, magnesite or dolomite which consists of a fuel oil-fired furnace provided with two parallel shafts. Each of the shafts communicate with one or more gas bypass passages one of which is common to both shafts, which enables the downwardly moving charge to be isolated from the upwardly flowing furnace gases at a point in the furnace where a high degree of temperature uniformity is necessary in the charge.

PATENTEDAUB24|971 3301.3.

' sum 1 0F 5 EDWARD CHARLES JOPE, Inventor BY, mam/m. EELLL-l Attorneys PATENTED AUB24I97| 3.601, 377

SHEET 5 [IF 5 EDWARD CHARLES JOPE, Inventor Attorneys SHAFI FURNACES 1. Field of the Invention The field of the invention is believed to be encompasses in class C04 of the International Classification of Patents. 2. Description of the Prior Art Shaft kilns have been in use for many years for calcining limestone of a relatively large stone size, i.e. 3-7 inches in diameter. However, the known apparatus are not satisfactory for calcining stone of smaller dimensions than this. In addition the product is not uniformly calcined, since heating gases cannot readily flow through the fine material, and channelling occurs. This reduces the accuracy to which the calcining process can be controlled and some of the lime emerges dead burned.

Nevertheless, approximately 30 percent of quarried limestone is in the size rangeof 1-3 inches in diameter. Also there is commercial demand for calcined lime of this size, for example, in modern steel making processes, in which the ideal size for added lime is up to 40 mm. in diameter.

A number of attempts have been made to produce a kiln satisfactory for calcining small stone limestone (of 1-2 inches in diameter), notably rotating furnaces, crossflow and rotating hearth furnaces, but these have proved to have high capital,

cost and poor fuel efficiency.

SUMMARY OF THE INVENTION The present invention provides a process for calcining a mineral, which process comprises charging the mineral to a furnace, heating the mineral in a preheating zone of the furnace, passing the preheatedmineral into a preheat soaking zone as hereinafter defined, in whichthe temperature of the mineral becomes substantially uniform, passing the resulting mineral to a burning zone in which the mineral is contacted with hot combustion gases from a fuel gasifier or burner and in which the mineral undergoes a chemical change with evolution of gas, withdrawing the evolved gas-and combustion gas from the furnace via a gas passage, whichmay be in the form of a chamber, and which, for example, is adjacent to the forms part of the preheat soaking zone, passing said gas through the preheating zone, cooling the calcined products in a cooling zone by the injection of air, and withdrawing the calcined product.

The present invention also provides apparatus for calcining a mineral which apparatuscomprises a furnace provided with at least one substantially vertical shaft lined with refractory material, with at least one inlet means for the mineral and with at least one discharge means for calcined mineral, which shaft is provided with at least one fuel supply means, with atleast one gas exit means and with at least one gas passage bypassing a preheat soaking zone as hereinbefore defined.

By the term preheat soaking zone as used herein is meant the zone in the shaft below the preheat zone where the charge is heated initially and above the burning zone where the hot charge is calcined.

A further aspect of the present invention provides a method of operating an apparatus of the invention having at least two shafts wherein gases from the combustion and calcination processes in the furnace are passed into a gas passage common to at least two shafts and are subsequently removed via the gas exit means of one shaft or via both shafts,either alternately or simultaneously. i g

The 4s to be calcined is usually limestone, but may also be dolomite, magnesite, gypsum or any other calcinable material. Limestone with an average stone size of 20 mm. to 45 mm. is preferred.

The apparatus of the invention comprises a shaft, whose preferred cross section is rectangular, or rectangular with round corners, or elliptical. The most preferred cross section is rectangular with two sides longer than the other two. The width of the shaft may be as narrow as is consistent with an even and steady flow of material, since the temperature differences ina horizontal direction will then be minimized. The shaft may be lined with any suitable refractory material, for example the lower temperature portions of the shaft may be lined with alumina, and the higher temperature portions with rammed, tarred dolomite. v The width of the shaft may be gradually increased to allow for expansion of the stone as it becomes heated, and also to avoid physical blockage. It should be noted that when small stone calcinable material is processed, heating up of the charge, and hence its expansion, may be more rapid than that of a normal sized charge.

The calcinable material may be fed to the shaft by means of a hopper, and since the apparatus will be operating at a pressure different from atmospheric pressure, it is preferred that the hopper is sealed.

The discharge means forthe calcined material preferably operates continuously, so that a uniform flow of material down the shaft occurs.

The fuel supply means of the apparatus may comprise any number of gas ports or oil gasifying/burning units. However it is preferred that the fuel supply means should be Catagas" heavy fuel oil gasifiers as described in the applicants British Pat. No. 904,435. In the apparatus of the present invention most of the combustion air is introduced to the central core of the shaft very close to the discharge units, so that it is warmed by the outflowing material before reaching the fuel supply means. Air may also be introduced elsewhere, according to necessity.

Gas exit means are provided at the top of the apparatus and it has been found useful, as hereinafter described, to provide valve means in the exit means, so that strict control over the exit gases can be maintained. The gas exit means is connected to a powerful fan, capable of producing a substantial reduction in the pressure at the top of the shaft.

The gas may also be extracted through an outlet provided at the level of the preheat soaking zone, in a crossflow manner.

Means-may be provided for introducing into the apparatus recirculated waste gases.

A feature of the apparatus of the present invention is the provision of a gas passage in the shaft at a level between that of the fuel supply means and the gas exit means. Solid material cannot pass into the gas passage. The walls of the passage are preferably constructed of a refractory or other suitable material capable of conducting sensible heat, provided by the gas, to the solid material in the preheat soaking zone.

The apparatus of the invention may comprise a plurality of shafts wherein at least one gas passage is common to at least two of the shafts. Preferably the gas passage is formed in the dividing wall between the shafts.

In a preferred embodiment, the invention provides an apparatus comprising two shafts as described above, constructed with their longer sides adjacent and a gas passage common to both shafts.

BRIEF DESCRIPTION OF DRAWINGS An example of the invention is shown in the accompanying drawings in which:

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENT Referring now particularly to FIG. 1, the apparatus consists of two tapered shafts l and 11 formed by a structure of refractory brick. The structure is partially supported on two foundation blocks 12 and 13 and partially by a steel framework of Double I or box section girders 14 which framework is covered with sheet steel. Two delivery cones 15 and 16 extend from the shafts between the foundation blocks 12 and 13 so that they are easily accessible. Each shaft is fed by a hopper 17 (shown in part) supported on top on the framework 14 directly above the shaft.

The shape of the refractory structure is complex but symmetrical about a line bisecting the section shown in FIG. 1 and so it will be described with reference to half of the section only. It should be understood that there are corresponding services to the right shaft of the furnace shown in the drawing.

Referring now to FIGS. 2 and 3 as well as FIG. 1, at a level slightly above the top of foundation block 12 there is located an air main 18 which surrounds a steel jacket 19 of the furnace. Connected to the main and passing through the central partition of the furnace is an air pipe 20. As can be seen from FIGS. 1 and 2 the pipe 20 is connected to the shaft by eleven supply pipes 21 and as can be seen from FIGS. 1 and 3 there is a similar connection between the main l8 and the shaft via supply pipes 22.

At a higher level in the furnace a channel 23 of rectangular cross section pierces the central partition and is connected to the shaft by fifteen subchannels 24, again of rectangular cross section, as shown in FIGS. 2.

In the external wall of the shaft at a level slightly above that of the subchannels 24 are set four Catagas fuel oil gasifiers 25 of the type described in British Pat. No. 904,435 the mouth of which opens into the shaft. Consequently the walls of refractory structure slope outwards at this point. Associated with the oil gasifier units is an air main 26 which is connected to each gasifier unit 25 by a primary air pipe 27. Also associated with the gasifier units 25 is a gas main 28 which is connected to each unit by a pipe 30 and to the shaft immediately above each unit by a gas supply tube 31. The entry into the shaft of the tubes 31 can be seen in FIG. 3. The central partition is cut to form an arcuate channel 32 opposite the burner units as a support for the refractory material above.

Above the arcuate channel 32 and at a level corresponding to that of the air pipe 27 a combined air and gas pipe 33 runs through the central partition and is connected with the shaft via 1 l delivery tubes 34 which are readily seen from FIGS. 1 and 2.

A joint 35 is provided in the external refractory structure of the furnace at a level slightly above that of the air and gas pipe 33, to allow for expansion and contraction.

Up to this level of the furnace the two shafts have been separate and independent but here a set of eleven gas passages 36 set in the central partition, communicates with each shaft by three sets 37, 38 and 39 of eleven channels of rectangular cross section. The passage 36 is provided with an inspection door 40. Also a combined air and gas pipe 41 is set into the central partition at the bottom of the gas passage and is connected to each of the channels 37 by l I T-shaped parts 42.

At the level of the gas passage 36 the external walls of the shafts are each provided with a set of 10 gas bypass ducts 43 connected at one end to the shaft by entry channels 44 and at the other end by 10 exit channels 45, both ofwhich are clearly shown in FIG. 3. The exit channels 45 are controlled by baffles 46 and at this level the refractory structure is provided with supports 51 and 57. The former, whichis rectangular in cross section may be used for heating incoming air and/or gas.

Disposed in the central partition slightly above the level of the gas exit channels 45 is a preheat channel 48 which is connected externally of the furnace to the various air and gas pipes used lower down the furnace through the conduit 51. From this level the shape of the refractory structure is such that the shafts broaden out until they meet two orifices 49 from the hopper 17. At the sides of the furnace the refractory structure is pierced by vents 53 communicating with the exhaust pipes 50 which lead to a powerful fan (not shown) and to the atmosphere.

Referring now to FIG. 4 it can be seen that the central portion of the refractory structure is pierced by the l 1 gas passages 36 communicating with the shafts by the channels 39 and that the outer walls of the structure are pierced by the IQ gas bypass tubes 43. The shafts 10 and 11 are also shown, as is the steelwork 51 and part of the double I section framework 14.

Referring now to FIG. 5 of the accompanying drawing the hoppers 17 are shown in section and the framework 14 and hopper supports 52 in plan.

Referring now to FIG. 6 of the accompanying drawings the exhaust pipes 50 are shown in section and the dust hopper 54 in plan.

In a preferred method of operating the apparatus described above, lime of small stone size 20-45 mm. in diameter, is cleaned for instance by flotation, washing with water or by passage over an efficient vibratory screen to remove dirt and fines and is fed to the hopper 17 which may feed material to the shaft continuously.

The material is then dried and preheated by warm gases which filter through from lower down the shaft to the gas exit means that is, to the exhaust pipes 50. It is preferred that the material reaches a temperature of at least 830 C. in this preheating zone 56 which extends down the shafts about as far as the baffle 55, from where it passes to a preheat soaking zone 57 which extends approximately from the baffie 55 to the joint 35.

The preheat soaking zone includes the narrowest part of the shaft, since it is at this level that the gas passages 35 are constructed. Gases from lower down the shaft do not pass through the material, but pass either through the gas passage, or through the gas bypass tubes 43 and so onto the preheating zone. This allows the preheated material in the preheat soaking zone to come to equilibrium with the surroundings and to attain a good degree of temperature uniformity. The absence of gas in this zone also eliminates the bad effects of channelling," i.e. the nonuniform passage of gas through the material, which is one of the most difficult problems in conventional small stone-burning processes, and which leads to uneven calcining in the burning region of the furnace. It is preferred that the temperature of the gases passing through the gas passage and/or the bypass tubes should be at least 950 C., since at this temperature any free carbon remaining in the gases will be oxidized when it comes into contact with the preheated air which may be introduced into the passage as required. 950 C. is also the minimum temperature of the gas which will both raise the incoming stone to 830 C, and keep the stone in the preheat soaking zone at a temperature of at least 830 C. The material may be heated by the hot refractories of the gas passage. The calcinable material emerges from the preheat soaking zone at a uniform rate, at a uniform temperature, and unaffected by channelling, passes to the burning zone.

In the burning zone which extends from the joint 35 to the bottom of the Catagas units 25, the calcinable material is burnt, for example, limestone is burnt to lime, with the evolution of gas. Burning is achieved by the oxidation of a fuel, for example heavy fuel oil, with air or oxygen, and the hot exhaust gases pass with the evolved gas to the preheat soaking zone and preheating zone described above. The temperature in the burning zone may be from about 1,100 C., to about l,240 C, and is preferably about l,l50 C. The calcined material then passes out of the shaft via the discharge-cones, exchanging its heat with the cool air for combustion in a heat exchanger, not shown.

The temperature of the calcinable material in the various zones of the furnace may be controlled by the addition of air or waste furnace gases through some or all of the ducts 18, 20, 24, 26, 27, 33, 42 as well as by regulation of the amount of gas The introduction of combustion air at various levels in the furnace ensures complete combustion of fuel within the charge. Also the possibility of adding combustion air to the central gas passages 36 means that any carbon particles which may pass up from the burning zones may be oxidized without interfering with the ratio of air to fuel in the burning zones. The result of this is that only an absolute minimum of excess air need be employed in the burning zones, which is very desirable in calcination processes. Another result is that smoke emission will be reduced.

Altering the flow of furnace gases by means of the bypass ducts 43 will permit the temperatures in the preheating and preheat soaking zones to be controlled. This may be done simuitaneously for both shafts of the furnace or for one shaft only. Bypassing gas in this way will also reduce heat radiation from the stone charge through the furnace walls to atmosphere as the charge passes through the preheat soaking zones.

In one method of performing the process in the preferred apparatus of the invention, the furnace gases are withdrawn from the gas passages through first the preheating zone of the one shaft of apparatus, and then through that of the other. The direction of flow may be switched about every 30 minutes, or there may be a gradual change from total flow of the gas through one preheating zone to total flow through the other. The gases may also, of course, be withdrawn through both zones equally and simultaneously.

The process as carried out in the preferred embodiment of the invention will be further described in the following example.

EXAMPLE The apparatus as described above with reference to the drawings is operated as follows:

A charge of small stone limestone is first cleaned of quarry dirt and fine chippings by washing or by passing it over efficient vibratory screens. It is then passed to the hopper of the kiln from where it is fed into one or other of the shafts at a rate of 130 tons per day and moves towards the discharge cones at a rate of 34 per hour. Heavy fuel oil is fed tothe Catagas units at the rate of pounds per minute and air is fed to the apparatus at a rate of 2,170 cubic feet perminute at normal temperature and pressure.

Under these conditions, the temperature in the burning zones is 1,200 C., and the stone remains in the zone for about 1 hour. Sixty-four tons per day of calcined lime is produced.

The combustion gas, excess air and evolved CO pass at therate of 2,470 cubic feet per minute at normal temperature pressure from the burning zones to the gas passages, which they reach at a temperature of950 C. minimum.

As stated above, the gas exit means is provided with movable shutters and with valves. By means of the latter, the gas entering the gas passages may be withdrawn through first one preheating zone, and then the other, alternately, the direction offlow may be switched at desired intervals.

The process and apparatus described above have a number of advantages. For instance, the Catagas Units may be pressurized in relation to the combustion air pressure, and these pressures may be varied without creating any major problems in draught balance. In other words primary and secondary combustion air may be pushed"'through the burning zone at any required volume and pressure. This hasthe advantage that the power of the main exhaust fan may be reduced, and that the process is flexible enough to accommodate any changes brought about by a fluctuation in stone size.

segregating into various dimensions in the charge is much less with two narrow shafts fed by separate columns of stone, than with a single wider shaft.

ln'times of slack production, one shaft of the apparatus may be'shut down while the other continues to operate with no loss of efficiency.

The controlled channelling of the hot kiln gases to bypass the preheat soaking zone, and the introduction of this zone permits a greater active kiln length for an equal exhaust fan power. Gas friction is eliminated in this zone.

The wide gas ports at the top of the burning zone reduce the amount oflime dust deposited and may even be self-cleaning.

The invention also includes calcined minerals whenever prepared in the apparatus or by the process described above.

Iclaim:

l. A shaft furnace for converting a mineral to a calcined mineral with the production of gas, said furnace comprising two substantially parallel and vertical shafts of substantially rectangular cross section lined with refractory material said refractory material forming a single dividing wall between said shafts and three external walls of each shaft, each shaft being provided close to its upper end with an inlet means for said mineral and with an exit means for said gas and close to its lower end with an exit means for said calcined mineral, a heavy fuel oil gasifyer protruding into each shaft between said inlet means and said calcined mineral exit means, each shaft being provided with means for the introduction of recirculated furnace gas and air at various levels in the furnace wherein a gas passage common to both shafts is constructed in said dividing wall, said gas passage bypassing a preheat soaking zone in each of said shafts and being provided with means for the introduction of a free oxygen containing gas and wherein the two refractory external walls opposite said dividing wall are each provided with a gas passage which bypasses a preheat soaking zone in each of said shafts.

2. A shaft furnace for converting a mineral to a calcined mineral with the production of gas, said furnace comprises a substantially vertical shaft lined with refractory material, said shaft being provided close to its upper end with an inlet means for said mineral and with an exit means for said gas and close to its lower end with .an exit means for said calcined mineral,

The narrowness of the shafts of the preferred apparatus of I fuel-burning means opening into said shaft between said inlet means and said calcined mineral exit means, a soaking zone in said shaft, at least one gas passage bypassing said soaking zone, the entry point for said gases into said gas passage being at a level in said shaft above the calcining zone and the exit point from said gas passage for said gases to reenter said shaft being at a higher level in said shaft, said higher level marking the upper boundary of said soaking zone and the lower boundary of the preheating zone.

3. A shaft furnace according to claim 2 comprising at least two gas passages, the gas entry and exit points of at least one of said gas passages being respectively at a level in the kiln above the gas entry and exit points of at least one of the remaining gas passages.

4. A shaft furnace according to claim 2 wherein there are two of said shafts and at least one of said gas passages is constructed in a dividing wall between said shafts to communicate with both shafts and bypasses a soaking zone in each of said shafts.

5. A shaft furnace according to claim 2 wherein means for the controlled introduction ofa free-oxygen-containing gas is provided in said gas passage.

6. A shaft furnace for converting a mineral to a calcined mineral with the production of gas, said furnace comprising two substantially parallel and vertical shafts of substantially rectangular cross section lined with refractory material, said refractory material forming a single dividing wall between said shafts and three external walls of each shaft, each shaft being provided close to its upper end with an inlet means for said mineral and with an exit means for said gas and close to' its lower end with an exit means for said calcined mineral, a heavy fuel oil gasifyer opening into each shaft between said inlet means and said calcined mineral exit means, each shaft being provided with means for the introduction of recirculated furnace gas and air at various levels in the furnace, a soaking zone in each of said shafts, a gas passage common to both shafts in said dividing wall bypassing said soaking zone, means for the introduction of a free oxygen containing gas into said gas passage and two refractory external walls opposite said dividing wall each provided with a gas passage bypassing a preheat soaking zone in each ofsaid shafts.

7. A shaft furnace according to claim 6 wherein the gas entry and exit points of the gas passages constructed in the external walls are respectively at a level in the kiln above the gas entry and exit points of said gas passage constructed in the dividing wall.

8. A continuous process for calcining a mineral comprising the steps of a. charging said mineral to a shaft furnace.

b. heating said mineral in a preheating zone of said furnace by contacting said mineral with hot gas rising through said furnace.

c. passing the heated mineral to a soaking zone of said furnace in which contact with said hot gas is prevented.

d. burning said mineral to a calcined mineral in a burning zone ofsaid furnace with the production ofgas.

e. cooling said calcined mineral in a cooling zone of said furnace.

f. withdrawing said calcined mineral from the bottom of said furnace wherein the gases produced in said burning zone are continuously withdrawn through the mass of mineral in the furnace except in the soaking zone of the furnace which the gas bypasses by means of at least one gas passage.

9. A process according to claim 8 wherein the depth of the soaking zone is controlled bypassing a proportion of the gases through at least one gas passage, the gas entry and exit points of which gas passage are respectively at a level in the kiln above the gas entry and exit points of at least one of the remaining gas passages.

10. A process according to claim 8 wherein the mineral is limestone of from about 1 inch to 2 inches in diameter. 

1. A shaft furnace for converting a mineral to a calcined mineral with the production of gas, said furnace comprising two substantially parallel and vertical shafts of substantially rectangular cross section lined with refractory material said refractory material forming a single dividing wall between said shafts and three external walls of each shaft, each shaft being provided close to its upper end with an inlet means for said mineral and with an exit means for said gas and close to its lower end with an exit means for said calcined mineral, a heavy fuel oil gasifyer protruding into each shaft between said inlet means and said calcined mineral exit means, each shaft being provided with means for the introduction of recirculated furnace gas and air at various levels in the furnace wherein a gas passage common to both shafts is constructed in said dividing wall, said gas passage bypassing a preheat soaking zone in each of said shafts and being provided with means for the introduction of a free oxygen containing gas and wherein the two refractory external walls opposite said dividing wall are each provided with a gas passage which bypasses a preheat soaking zone in each of said shafts.
 2. A shaft furnace for converting a mineral to a calcined mineral with the production of gas, said furnace comprises a substantially vertical shaft lined with refractory material, said shaft being provided close to its upper end with an inlet means for said mineral and with an exit means for said gas and close to its lower end with an exit means for said calcined mineral, fuel-burning means opening into said shaft between said inlet means and said calcined mineral exit means, a soaking zone in said shaft, at least one gas passage bypassing said soaking zone, the entry point for said gases into said gas passage being at a level in said shaft above the calcining zone and the exit point from said gas passage for said gases to reenter said shaft being at a higher level in said shaft, said higher level marking the upper boundary of said soaking zone and the lower boundary of the preheating zone.
 3. A shaft furnace according to claim 2 comprising at least two gas passages, the gas entry and exit points of at least one of said gas passages being respectively at a level in the kiln above the gas entry and exit points of at least one of the remaining gas passages.
 4. A shaft furnace according to claim 2 wherein there are two of said shafts and at least one of said gas passages is constructed in a dividing wall between said shafts to communicate with both shafts and bypasses a soaking zone in each of said shafts.
 5. A shaft furnace according to claim 2 wherein means for the controlled introduction of a free-oxygen-containing gas is provided in said gas passage.
 6. A shaft furnace for converting a mineral to a calcined mineral with the production of gas, said furnace comprising two substantially parallel and vertical shafts of substantially rectangular cross section lined with refractory material, said refractory material forming a single dividing wall between said shafts and three external walls of each shaft, each shaft being provided close to its upper end with an inlet means for said mineral and with an exit means for said gas and close to its lower end with an exit means for said calcined mineral, a heavy fuel oil gasifyer opening into each shaft between said inlet means and said calcined mineral exit means, each shaft being provided with means for the introduction of recirculated furnace gas and air at various levels in the furnace, a soaking zone in each of said shafts, a gas passage common to both shafts in said dividing wall bypassing said soaking zone, means for the introduction of a free oxygen containing gas into said gas passage and two refractory external walls opposite said dividing wall each provided with a gas passage bypassing a preheat soaking zone in each of said shafts.
 7. A shaft furnace according to claim 6 wherein the gas entry and exit points of the gas passages constructed in the external walls are respectively at a level in the kiln above the gas entry and exit points of said gas passage constructed in the dividing wall.
 8. A continuous process for calcining a mineral comprising the steps of a. charging said mineral to a shaft furnace. b. heating said mineral in a preheating zone of said furnace by contacting said mineral with hot gas rising through said furnace. c. passing the heated mineral to a soaking zone of said furnace in which contact with said hot gas is prevented. d. burning said mineral to a calcined mineral in a burning zone of said furnace with the production of gas. e. cooling said calcined mineral in a cooling zone of said furnace. f. withdrawing said calcined mineral from the bottom of said furnace wherein the gases produced in said burning zone are continuously withdrawn through the mass of mineral in the furnace except in the soaking zone of the furnace which the gas bypasses by means of at least one gas passage.
 9. A process according to claim 8 wherein the depth of the soaking zone is controlled by passing a proportion of the gases through at least one gas passage, the gas entry and exit points of which gas passage are respectively at a level in the kiln above the gas entry and exit points of at least one of the remaining gas passages.
 10. A process according to claim 8 wherein the mineral is limestone of from about 1 inch to 2 inches in diameter. 